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United States Patent |
5,654,341
|
Struewing
|
August 5, 1997
|
Low pH skin-treatment composition
Abstract
A low pH skin-treatment composition employs cationic surfactants generally
known to be skin irritants and a fatty acid, together with a hydrocarbon
component in an oil-in-water emulsion. The addition of fatty acid to the
cationic surfactant appears to form a complex which prevents the cationic
surfactant from irritating the skin and at the same time substantially
reduces pH of the composition. The resulting composition is not skin
irritating, and reflects a pH of about 2-4. The composition may be used to
restore pH of skin after exposure to alkaline compositions, and as a
vehicle for the administration of active agents having an acidic pH.
Inventors:
|
Struewing; Sharon (Cincinnati, OH)
|
Assignee:
|
The Andrew Jergens Company (Cincinnati, OH)
|
Appl. No.:
|
420060 |
Filed:
|
April 11, 1995 |
Current U.S. Class: |
514/642; 514/762; 514/789; 514/975 |
Intern'l Class: |
A61K 031/14; A61K 031/01 |
Field of Search: |
514/642,762,789,975
424/70.27,70.28
|
References Cited
U.S. Patent Documents
Re29814 | Oct., 1978 | Snyder | 514/772.
|
3818105 | Jun., 1974 | Coopersmith et al. | 514/789.
|
4168144 | Sep., 1979 | Curry et al. | 8/426.
|
4252695 | Feb., 1981 | Homma et al. | 252/547.
|
4315912 | Feb., 1982 | Kalopissis et al. | 424/70.
|
4369250 | Jan., 1983 | Gindler | 435/18.
|
4710314 | Dec., 1987 | Madrange et al. | 252/117.
|
4711776 | Dec., 1987 | Suzuki et al. | 424/70.
|
5019376 | May., 1991 | Uick | 424/70.
|
5143518 | Sep., 1992 | Madrange et al. | 8/405.
|
5248445 | Sep., 1993 | Rizvi et al. | 252/174.
|
Primary Examiner: Weddington; Kevin E.
Attorney, Agent or Firm: Oblon, Spivak, McClelland, Maier & Neustadt, P.C.
Claims
What is claimed is:
1. A low pH skin-treatment composition, which comprises the following
components:
(a) a cationic surfactant;
(b) a fatty acid;
(c) an oleaginous hydrocarbon comprising at least one member selected from
the group consisting of hydrocarbon oils, hydrocarbon waxes, silicon oils,
non-hydrocarbon waxes, fatty acid esters and mixtures thereof; and
(d) water,
said cationic surfactant, fatty acid and hydrocarbon components being
present in amounts sufficient to provide a non-skin irritating emulsion
with pH of about 2-4, with water constituting the balance of said
composition.
2. The composition of claim 1, wherein said cationic surfactant component
is present in amounts of 0.1-30 percent by weight, based on the weight of
the composition, said fatty acid is present in amounts of 0.1-30 percent
by weight based on the weight of the composition and said hydrocarbon
component is present in amounts of 0.1-50 percent by weight based on the
weight of the composition.
3. The composition of claim 1, wherein said cationic surfactant comprises a
monoalkyl or dialkyl quaternary ammonium compound.
4. The composition of claim 3, wherein said cationic surfactant is selected
from the group consisting of cetrimonium bromide, behentrimonium chloride,
distearyldimmonium chloride, tricetylmonium chloride, Quaternium 52,
methyl bis(polyethoxyethanol) coco-ammonium chloride, palmitoylethyl
hydroxyethylmonium methylsulfate, dimethyl stearyl benzyl ammonium
chloride and mixtures thereof.
5. The composition of claim 1, wherein said fatty acid is a saturated fatty
acid of the formula C.sub.n H.sub.2 CO.sub.2 with n.ltoreq.9 and
unsaturated fatty acids of the formula C.sub.n H.sub.2n-x CO.sub.2 with
n.ltoreq.21 and x an even integer from 2-6.
6. The composition of claim 5, wherein said fatty acid is selected from the
group consisting of stearic, palmitic, oleic, linoleic, linolenic acids
and mixtures thereof.
7. The composition of claim 1, wherein said cationic surfactant component
comprises at least one of distearyldimmonium chloride and
cetyltrimethylammonium bromide and said fatty acid component comprises
stearic acid.
8. The composition of claim 1, wherein said composition further comprises
at least one of a humectant, keratolytic agent, water-soluble polymer,
preservative, moisturizing agent, coloring, and a fragrance.
9. The composition of claim 8, wherein said humectants are present in an
amount of 0-10 percent by weight, based on the total weight of the
composition, and said keratolytic agent, water-soluble polymer,
preservative, moisturizing agent, coloring or, fragrance are each present
in an amount of 0-5 percent by weight, based on the total weight of the
composition.
10. A method of softening skin, comprising applying the composition of
claim 1 to said skin in an amount sufficient to provide emollient
properties, without skin irritation.
11. A method of reducing skin pH from a value above 4, by applying the
composition of claim 1 thereto, in an amount sufficient to reduce the pH
of said skin.
12. A method of returning skin pH to a value of 4-6 subsequent to exposure
to alkaline materials which raise the pH of said skin to a value above 6
upon exposure to said skin, comprising applying the composition of claim 1
to said skin in an amount sufficient to reduce the pH of said skin to a
value between 4 and 6.
13. A method of administering keratolytic agents having an acidic pH to
skin of a patient by topical administration comprising admixing said
pharmaceutical agent with the composition of claim 1, and applying the
resulting admixture to said skin of said patient in an amount sufficient
to provide a keratolytically effective amount of said agent.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention pertains to a composition especially adapted for treatment
of mammalian skin, in particular, human skin. The composition is
characterized by a low pH, and suitable for topical administration to the
skin.
2. Background of the Prior Art
Mammalian skin, and human skin in particular, has a normal pH generally in
the range of 4-6. A variety of agents that are commonly encountered and
used for health and/or beauty care, including soaps and detergents, tend
to raise skin pH above its general value. The skin acts as a general
buffer zone or protectant against outside agents, and a change in the pH
may hamper or limit the ability of the skin to provide its protective
functions. Recovery of pH in the skin, after exposure to soaps and the
like, is particularly retarded in certain classes that might be considered
"at risk".
One common form of skin-treatment compositions can be characterized as
oil-in-water emulsions, in which an oleaginous phase, predominantly
hydrocarbon, generally present in a minor amount, is prepared in an
emulsion with water or an aqueous phase. To maintain the stability of such
emulsions, a surfactant is commonly employed. Nonionic and anionic
surfactants are familiar to those of skill in the art. In general cationic
surfactants, such as cetyltrimethylammonium bromide, although sometimes
used as such as cetyltrimethylammonium bromide, although sometimes used as
active agents in anti-bacterial preparations, are not conventionally used
in skin treatments, due to their irritation potential. Indeed,
cetyltrimethylammonium bromide (CTAB) is considered a strong irritant to
the skin according to the Federal Health and Safety Association (FHSA). A
rare exception to the strongly irritating nature of this class of
surfactants is distearyldimmonium chloride, which has a markedly lower
irritant characteristic, and is currently used in skin care products such
as EVERSOFT.TM. (Andrew Jergens Company), CUREL.TM. and SOFTSENSE.TM.
(Bausch & Lomb) and AVEENO.TM. (S. C. Johnson). Nonetheless, wider scale
use of cationic surfactants in skin care emulsions would be desirable, for
a variety of reasons. Cationic surfactants carry a positive charge. This
is advantageous in light of the negatively charged characteristic of skin.
Moreover, nonionic surfactants do not thicken emulsions, and require
specific preservatives for use. Anionic surfactants carry a negative
charge. Anionic surfactants have been demonstrated to cause damage to the
stratum corneum barrier. Indeed, sodium lauryl sulfate, an anionic
surfactant, is often used as a positive control in irritation tests.
Due to the high irritant character of cationic surfactants, their use in
the prior art in compositions for topical application has been limited,
principally, to compositions for the treatment of hair. Hair, like skin,
is substantially negative, and thus substantively attractive to the
cationic surfactant. Among the prior art directed to such compositions
include U.S. Pat. No. 5,019,376, Uick, described as being particularly
useful as hair conditioning compositions, with a pH of 2-5, and U.S. Pat.
No. 4,252,695, Homma et al, with a pH of 5, and again devoted to hair
conditioning. Other hair conditioning art includes U.S. Pat. No.
4,711,776, Suzuki et al, U.S. Pat. No. 5,143,518, Medrange et al, and U.S.
Pat. No. 5,248,445, Rizvi et al. Hair colorants are described in U.S. Pat.
No. 4,168,144, Curry et al. Medrange and Curry are directed to
combinations of cationic surfactants and fatty acids which combinations
exhibit basic pH values and are unsuitable for the treatment and
preservation of the natural pH of human skin. None of the references
identified discloses a low pH skin conditioning treatment. U.S. Pat. No.
4,710,314, Medrange et al, is also directed to a composition using
cationic silicone polymers, but not the cationic surfactants of the
claimed invention. This case is again directed to hair treatment
compositions.
A different type of skin-conditioning composition, which is not an
emulsion, is addressed in International Publication WO93/07856, Deckner et
al, which is directed to a polyacrylamide gel, specifically indicated to
be nonionic. The reference specifically states that surfactants are not be
used, page 10, lines 28-31. The reference does establish a variety of
reasons for preparing low pH skin-treatment compositions, which include,
in addition to augmenting or enhancing the buffering action of skin in
response to alkaline materials, using the low pH formulations as better
vehicles for skin-active agents which are applied topically, such as
keratolytic agents, anti-acne agents and the like. There are a variety of
pharmaceuticals which exhibit low (acidic) pH values, and thus are better
suited to a low pH carrier.
Fatty acids (saturated and unsaturated) constitute a class of compounds
commonly used in many skin care products as opacifiers, thickeners and
emulsion stabilizers. The general structure for saturated fatty acids is
C.sub.n H.sub.2n CO.sub.2, while unsaturated fatty acids contain at least
one double bond, C.sub.n H.sub.2n-x CO.sub.2. In general saturated fatty
acids with more than 10 carbon atoms are solid at room temperature, and
may be unsuitable for use in liquid emulsion skin care compositions.
Unsaturated fatty acids are generally liquid at room temperature. Stearic
acid is a widely used saturated fatty acid, generally used in the form of
sodium stearate. This compound finds wide application as a soap
emulsifier. In general, fatty acids appear as mixtures of other closely
related acids. Commercially available stearic acid contains a mixture of
C.sub.18 and C.sub.16 acids. Similarly, palmitic acid is a mixture of
C.sub.16 and C.sub.14 acids.
Accordingly, it remains a goal of those of skill in the art to provide a
low pH skin care composition that can be used to address or neutralize the
effects of alkaline materials such as soaps and detergents which cause the
skin pH to rise above normal skin pH of 4-6.
A further object of those of skill in the art is to find a method for using
cationic surfactants in skin care emulsions without causing skin
irritation. An additional object of the art is the provision of a skin
care composition with low skin irritation, low pH and employing fatty
acids as an agent therein.
SUMMARY OF THE INVENTION
The objects set forth above, and additional objects made clear by the
disclosure set forth below, are achieved by the provision of a low pH skin
care composition which includes a cationic surfactant, a saturated or
unsaturated fatty acid, and an oleaginous phase, generally a hydrocarbon,
such as mineral oil or the like. An emulsion is prepared, with the balance
being substantially water, or an aqueous phase based on water.
In its broadest embodiments, the invention requires sufficient cationic
surfactant to be present such that a stable emulsion is provided, and that
the overall composition exhibits a pH below 4, preferably 2-4. The fatty
acid component is incorporated in an amount sufficient to reduce or
suppress skin irritation caused by the presence of the cationic
surfactant, apparently by complexing with the cationic surfactant, forming
a bulky complex with less freedom in the active agents of the cationic
surfactant that irritate the skin and cause the reduction in pH. The
oleaginous or hydrocarbon phase is present in amounts sufficient to
provide good emollient and "feel" properties, with the balance being
water. Additional compounds, including humectants, dyes, perfumes, and
other components which effect the aesthetics of the emulsion prepared,
making it more suitable for application to the skin, may be added in
amounts that do not effect the basic low pH, low skin irritation and
emulsion characteristics of the composition.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 (graph 1) is a graphic demonstration of the effect of stearic acid
concentration on pH of a solution bearing a cationic surfactant
(distearyldimmonium chloride).
FIG. 2 (graph 2) graphically compares the changes in pH caused by the
addition of various fatty acids to a cationic surfactant-containing
composition.
FIG. 3 (graph 3) is a graphic representation of the change in pH caused by
the addition of fatty acid (stearic acid) to a commercial skin treatment
product, EVERSOFT.TM..
FIG. 4 (graph 4) is a graphic representation on the effects of the
application of inventive and non-inventive products on pH measured at the
skin surface of test subjects.
FIG. 5 (graph 5) is a graphic representation of the effect of treatment of
the skin with the products reflected in FIG. 4 on skin surface pH after
washing the skin with soap.
DETAILED DESCRIPTION OF THE INVENTION
The inventive skin care composition is an oil-in-water emulsion, and is
therefor based on an aqueous preparation from which an emulsion is
prepared. The aqueous preparation is preferably water. In addition to
water, there are three required components of the inventive emulsion: (1)
a cationic surfactant, (2) a fatty acid component and (3) a hydrocarbon
phase, such as mineral oil. The amounts of these three essential
components are not critical to the invention, provided the goals of low pH
(2-4), low skin irritation and stable emulsion properties are obtained.
Nonetheless, broadly stated, the cationic surfactant and fatty acid are
generally present in amounts of 0.1-30 percent, by weight, based on the
total composition. A preferred range is 0.1-10 percent, by weight. The
hydrocarbon phase is generally present in an amount of 0.1-50 percent, by
weight, based on the total composition. A preferred range is 0.5-30
percent, by weight. As the hydrocarbon phase is provided primarily as an
emollient, and to provide an acceptable feel and aesthetic texture and
appearance to the composition, the amount included will vary substantially
with the identity of the hydrocarbon composition. In its broadest
embodiment, these three components are present together with water.
cationic surfactants include quaternary ammonium compounds. These may be
monoalkyl, dialkyl or trialkyl, and are represented by Formulas I-III,
below. Monoalkyl and dialkyl formulations are preferred from the point of
view of solubility and stability.
##STR1##
a, b, c=Low molecular weight groups (e.g., CH.sub.3, CH.sub.2 --CH.sub.3,
H)
R=Hydrophobic group (e.g., C.sub.8 -C.sub.18 hydrocarbons)
X=Anionic counter ion (e.g., bromide, chloride, methylsulfate)
Suitable cationic surfactants were prepared in a simple emulsion whose
composition is reflected in Table A below.
TABLE A
______________________________________
A B
PERCENT PERCENT
INGREDIENT (WEIGHT) (WEIGHT)
______________________________________
Water 77.50 76.50
Glycerin 10.00 10.00
Polymer Jr 0.20 0.20
Ceteareth-20 0.80 0.80
Cetearyl Alcohol
2.00 2.00
Petrolatum 3.00 3.00
Dimethyicone 2.00 2.00
Mineral Oil 3.00 3.00
Stearic Acid 0.00 1.00
Cationic Surfactant
1.50 1.50
______________________________________
The pH of the emulsion was taken with and without the addition of stearic
acid (1.00 percent by weight). This formulation was used to determine the
pH of the various cationic surfactants at a concentration of 1.50 percent
by weight.
TABLE B
______________________________________
CATIONIC pH RESULTS
SURFACTANT A B
______________________________________
Cetrimonium Bromide 4.35 2.66
(monoalkyl)
Behentrimonium Chloride
5.45 3.82
(monoalkyl)
Distearyldimmonium 4.50 2.69
Chloride
(dialkyl)
Tricetylmonium Chloride
5.04 3.44
(trialkyl)
Quaternium 52 6.48 4.61
(phosphate ion)
Methyl Bis(polyethoxy
5.99 3.43
Ethanol)
Coco-Ammonium Chloride
(dialkoxy-alkyl)
Palmitoylethyl -- 2.94.sup.1
Hydroxyethylmonium
Methylsulfate
(methylsulfate ion)
Dimethyl Stearyl -- 2.40.sup.2
Benzyl Ammonium
Chloride
(Benzyl Quat)
______________________________________
.sup.1 Fatty Acid = isostearic acid
.sup.2 Stearic acid concentration = 2.5 percent
As noted, the cationic surfactant increases pH of the composition.
Accordingly, the amount of the cationic surfactant must be controlled to
avoid increasing pH above about 4, and similarly to avoid a skin
irritation. The amount of cationic surfactant is carefully controlled in
conjunction with the amount of fatty acid present. Nonetheless, a broad
range of 0.1-30 percent by weight, based on the total composition, is
generally suitable for the invention. A preferred range is 0.1-10 percent
by weight.
Suitable fatty acids for use in this invention include most conventional
fatty acids, that is, fatty acids which are liquid at room temperature.
This includes relatively low molecular weight saturated fatty acid,
C.sub.n H.sub.2n CO.sub.2, with n.ltoreq.9, and unsaturated fatty acids of
the formula C.sub.n H.sub.2n-x CO.sub.2. Preferred embodiments include
saturated fatty acids of 12-20 carbon atoms. Stearic, palmitic, oleic,
linoleic and linolenic acids (linoleic and linolenic including two and
three double bonds, respectively).
The fatty acid component is incorporated in an amount sufficient to reduce
the pH of the composition to a value of 2-4. As set forth below, the
addition of the fatty acid dramatically drops pH at relatively low
concentrations. At the same time, the fatty acid apparently complexes with
the cationic surfactant, to provide a bulky complex which reduces or
suppresses skin irritation. Without being bound by the explanation, it is
believed that the complexes are formed by ion pairing of the cation of the
surfactant with the carboxylate anion of fatty acid. As a result of the
pairing, HX is released, which lowers the pH of the emulsion. Although no
cosmetic formulations are described, this theory is supported by the
discussion in Cosmetics: Science and Technology, Volume 1 (Balsm et al,
Editors, 2d Edition). Thus, the fatty acid component is incorporated in
amounts of about 0.1-30 percent by weight. More preferably, the fatty acid
is present in amounts of 0.1-10.0 percent by weight, again based on the
weight of the composition.
The oleaginous or hydrocarbon phase is selected from any of a variety of
well-known oil or emollient components. The principal criteria is that the
emollient must be stable in a low pH environment. Representative oil
components include hydrocarbon oils, (e.g., petrolatum, mineral oil,
polydecene), silicone oils, (both volatile and non-volatile), fatty
alcohols, (e.g., cetyl, isostearyl, guerbet), waxes (e.g., hydrocarbon
waxes, meadowfoam seed oil) and fatty acid esters (e.g., isopropyl
myristate, myristyl myristate). The hydrocarbon phase is advantageously
used in an amount sufficient to give the composition an acceptable feel
and skin conditioning effect, generally in an amount of 0.5-30 percent by
weight, based on the total weight of the composition.
In addition to water, the aqueous phase, which constitutes the balance of
the composition, can include humectants, such as glycerin, propyleneglycol
or sorbitol, generally in an amount of 0-40 percent by weight, based on
the total weight of the composition. Other water soluble materials may be
added including keratolytic agents, water soluble polymers (e.g., polymer
JR), preservatives, moisturizing agents such as amino acids, colorings and
fragrances. These additives are generally incorporated in an amount of no
more than 5 percent by weight. Pharmaceutical agents, if incorporated, are
incorporated in an amount effective to achieve the pharmaceutical goal,
but generally, in no more than 5 percent by weight. Water is generally in
the range of 50-95 percent by weight.
EXAMPLES
The invention addressed herein is exemplified in the working and
comparative examples set forth below. These examples are not intended to
limit the invention in any way.
pH Reduction by Addition of Fatty Acid
A cationic surfactant-based emulsion, according to the formula set forth in
Table A above, was prepared, using distearyldimmonium chloride as the
cationic surfactant at a 1.5 percent weight concentration. As an exemplary
fatty acid, stearic acid was selected and added to the described
emulsions. The result is reflected in FIG. 1. As shown therein, dramatic
reductions in pH are reflected by the addition of relatively low amounts
of stearic acid, an amount of about 0.1 percent being sufficient to drop
the pH from a starting point above 5 to a value within the intended scope
of the invention, that is, between 2 and 4. This sharp drop in pH upon the
addition of fatty acid tends to confirm the theory advanced above, that
the fatty acid and cationic surfactant form a bulky complex which
liberates HX, resulting in a sharp drop in pH. Even upon the addition of
substantial amounts of fatty acid, the pH remains above 2, again clearly
indicating that the addition of the fatty acid alone is not responsible
for the pH drop.
The above-described experiment was also conducted with other representative
fatty acids. The results are depicted in FIG. 2. As shown in FIG. 2, the
addition of all fatty acids selected resulted in a drop of pH on the
addition of at least 0.5 weight percent to a value below 4. Bulkier
branched fatty acids did not cause the extreme drop in pH upon the
addition of minor amounts that was observed in connection with
straight-chain fatty acids. Accordingly, the fatty acid may be selected so
as to tailor the resulting desired pH within the identified range of the
invention.
As noted previously, certain commercial skin compositions are available
which include the cationic surfactant distearyldimmonium chloride. A
representative fatty acid, stearic acid, was added to the EVERSOFT.TM.
preparation and pH measured, with the results reflected in FIG. 3.
Clearly, even in commercial preparations designed for skin care with pH
above 4, modification within the scope of the invention disclosed and
claimed herein results in a dramatic drop in pH, such that the skin care
composition may be modified so as to improve the buffering action of the
skin by returning it to its normal pH after exposure to alkaline
materials.
Skin Irritation Tests
As noted above, a principal difficulty in employing cationic surfactants in
skin treatment compositions is the widely held belief that such
compositions are skin irritants. A test was conducted to measure the skin
irritation potential of the low pH formulations of the invention. The
protocol included Occlusive 25 mm Hilltop Chambers on the volar forearms.
The study was a three-day study with patches applied each day. The sites
tested were evaluated by the Transepidermal Water Loss Measurements
methodology, Simion et al, 1991. Transepidermal water loss was assessed
prior to the first patch and three hours after the last patch. TEWL
measures the rate at which water passes from the skin into the environment
(Van der Valk et al, 1984). Thus, skin sites with high TEWL rates in the
study indicate damage to the stratum corneum barrier. The cationic
surfactants tested included cetyltrimethylammonium bromide (CTAB) and
distearyldimmonium chloride (DSDC). The formulations employed in the skin
irritation study are reflected in Table 1. The formulations also contained
methyl and propyl paraben, as well as DMDM Hydantoin or Quaternium-15 as
preservatives. Quaternium-15 was used in Formula G because this was a
previously tested formula. Formulas A and B are without and with fatty
acid, respectively. Formula C contains a pH adjustor, lactic acid, to
study the low pH effect. Formulas D and E are monoalkyl cationic
surfactants with and without fatty acid, respectively. Formula F has a
base pH adjustor, sodium hydroxide, to bring the pH close to neutral.
Formula G is a monoalkyl cationic formula previously found to be
irritating. Results from the irritation study are given in Table II. TEWL
values are reported as degree of change from day 3-day 1.
TABLE I
______________________________________
A B C D E F
______________________________________
Water 66.60 66.50 65.46 66.60 66.60 62.60
Glycerin
10.00 10.00 10.00 10.00 10.00 10.00
Polymer 0.20 0.20 0.20 0.20 0.20 0.20
JR
DSDC 1.50 1.50 1.50 -- -- --
CTAB -- -- -- 1.50 1.50 1.50
Stearic -- 1.00 -- -- 1.00 1.00
Acid
Cetearyl
2.00 2.00 2.00 2.00 2.00 2.00
Alcohol
Mineral 3.00 3.00 3.00 3.00 3.00 3.00
Oil
Dimethi-
2.00 2.00 2.00 2.00 2.00 2.00
cone
Petro- 3.00 3.00 3.00 3.00 3.00 3.00
latum
Preserva-
0.70 0.70 0.70 0.70 0.70 0.70
tives
Lactic -- -- 1.14 -- -- --
Acid,
88%
NaOH -- -- -- -- -- 3.00
5% (wt)
Solution
Hydroxy-
-- -- -- -- -- --
ethyl
Cellulose
pH 4.50 2.69 2.15 4.34 2.66 7.53
______________________________________
TABLE II
______________________________________
A B C D E F
______________________________________
TEWL 0.70 .+-.
0.37 .+-.
1.02 .+-.
6.23 .+-.
2.03 .+-.
3.74 .+-.
DAY 3-
0.50 0.49 0.36 1.57 0.61 0.90
DAY 1
______________________________________
The results clearly indicate that the addition of the fatty acid lowers the
irritation observed. This is especially evident in Formula D, which
contains a monoalkyl surfactant, and is irritating, but, upon the addition
of stearic acid, Formula E, the irritation is lowered dramatically. All of
the reduced pH formulas were non-irritating. Formula C, pH 2.15,
containing lactic acid, did result in higher TEWL values than the other
low pH formulations containing stearic acid, demonstrating greater damage
to the stratum corneum barrier. The neutral adjusted fatty acid formula,
Formula F, exhibited irritation. The irritation is probably caused by the
strong attraction of sodium hydroxide to the stearic acid to form sodium
stearate. This attraction keeps the fatty acid from complexing with the
cationic surfactant, so the monoalkyl quaternary compound is not
completely shielded by the stearic acid carboxylate anion. Sodium lauryl
sulfate, SLS, 0.5 percent by weight, was used as the positive control. The
SLS sites exhibited the most severe irritation with a TEWL value of
46.80.+-.8.90.
Skin pH Studies
Skin surface pH studies were also conducted to study the effects of the low
pH formulations of the invention on skin pH. Measurements were made on the
volar forearm before and after application of the test product. Skin
surface pH was generated using a flat-surface electrode, calibrated in
standard buffer solutions at pH 4.0 and pH 7.0. The electrode was dipped
in deionized water before each measurement. Using a five-person panel,
four formulations were tested for effect on the skin surface pH of the
forearm. The formulas tested are listed in Table III. Formulas H and I are
cationic lotions with various levels of cationic surfactant and fatty
acid. Formulas J and K are nonionic emulsions. Formula J represents a
general lotion, while Formula K contains a lactic acid buffer system to
observe pH effects. The results are reflected in FIG. 4.
TABLE III
______________________________________
H I J K
______________________________________
Water 75.50 68.00 76.00 --
Glycerin 10.00 10.00 10.00 10.00
Cetearyl Alcohol
2.00 2.00 2.00 2.00
Stearic Acid 1.00 5.00 -- --
DSDC 1.50 5.00 -- --
Mineral Oil 10.00 10.00 10.00 10.00
Ceteareth-20 -- -- 2.00 2.00
Lactic Acid Buffer
-- -- -- 76.00
pH 2.72 2.19 5.18 4.19
______________________________________
Clearly, the cationic formulations, Products H and I, had the greatest
effect in lowering skin surface pH. Application of Product I, with higher
levels of cationic surfactant and fatty acid components resulted in the
lowest skin surface pH.
The same study was repeated, however, this time the formulations were
applied immediately after washing with soap, rather than application to
untreated skin. Volar forearms were washed for twenty second with a
commercial soap product, 1 percent solution pH=10.7, followed by a fifteen
second rinse of water. Measurements were made before and after soap
washing. The formulations were applied five minutes after rinsing to allow
time for the pH to be measured between washing and application of the test
composition. The results are reflected in FIG. 5.
Skin surface pH was elevated to 6.5-6.9 after washing with soap followed by
rinsing. The sites treated with the nonionic formulation, Product J, and
the control (no product application) gave the same pH. After thirty
minutes, the pH values were slightly above the initial pH values (prior to
washing). The low pH formulations of the claimed invention (H and I)
resulted in a substantial reduction in skin pH.
Accordingly, it is clear that the claimed invention provides a skin care
preparation which permits the use of cationic surfactants, whose prior use
has been limited by reason of their irritating properties. The addition of
a fatty acid results in a low pH emulsion which is non-irritating to the
skin. This is particularly surprising when it is noted that the pH of the
formulations of the claimed invention are typically below 3. Generally,
such compositions would be considered to be irritating to the skin, but
when tested were found non-irritating using patch testing. Skin surface
testing, both with and without soap-induced high pH values, show marked
decrease in pH values upon application of the claimed invention.
This invention has been disclosed in terms of a generic description,
specific embodiment, and by example. The embodiments and examples are not
intended to be limiting, and variations will occur to those of ordinary
skill in the art, without the exercise of inventive faculty. In
particular, other identities for cationic surfactants, fatty acids, and
the oleaginous phase beyond those specifically recited are well known to
those of skill in the art for use, separately, in skin care formulations,
and may be used herein without departing from the scope of the invention.
Similarly, weight percentages, additions of stabilizing and/or aesthetic
components and the like remain within the scope of the invention. These
and related variations remain within the scope of the invention, save as
excluded by recitation of the claims set forth below.
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